(1) Field of the Invention
The invention relates to field emission displays, and more particularly to structures and methods of manufacturing field emission displays with a built in ion pump, for eliminating outgassed material from within the display.
(2) Description of the Related Art
There is a growing need in the computer and electronics industries for thin, lightweight display panels. One application for such thin displays is for portable computers. The most commonly used display panel at the current time is the liquid crystal display (LCD), but because of the slow optical response time of the liquid crystal pixel and because of its relatively poor luminosity, other display technologies are being actively explored.
One such technology which has the potential to provide faster response times and increased brightness, while maintaining a thin profile and low power consumption, is the Field Emission Display (FED). As shown in FIG. 1, an FED typically consists of an array of small cold cathode electron emitters 18 mounted on a substrate 10, from which emitted electrons 20 are accelerated through an evacuated space to an opposing anode 24. The emitted electrons strike cathodoluminescent material 22 (phosphors), causing light 28 to be emitted, which may be viewed through a glass viewing surface 26 on which the anode and phosphors are mounted.
The array of very small, conically shaped electron emitters is electrically accessed by peripheral control and image forming circuits, using two arrays of conducting lines that from columns and rows. The array of column lines form the cathode contacts 12 on which the conical electron emitters are formed. The array of row conducting lines form gate electrodes 16 that are separated by a dielectric layer 14 from the column lines. The column lines 12 are formed on the substrate 10, and both the gate electrodes 16 and dielectric layer 14 have openings over the column lines, in which the emitters 18 are formed. The edges of the openings in the gate electrodes are in close proximity to the emitter tip, and function as the electrically addressable gate electrode 16, or control grid, for the individual electron emitters 18. A second, focusing, electrode (not shown) may be formed separated from and over the gate electrode, to provide narrower, more focused, electron streams as the electrons are emitted and accelerated toward the anode.
The proper functioning of the FED relies on maintaining an adequate vacuum within the cavity between the substrate on which the emitters are formed, and the transparent viewing plate. However, the vacuum can be degraded, during operation of the display, by outgassing from the materials from which the FED is fabricated. Outgassing primarily occurs when emitted electrons strike the anode/phosphor and cause trapped molecular gases or solids to be released. The outgassed materials not only degrade the vacuum but may also cause undesirable arcing within the FED, which can ultimately lead to destruction of the display.
To achieve and maintain a good vacuum, it is common practice in the vacuum tube industry to utilize a gettering material, such as barium (Ba), tantalum (Ta), titanium (Ti), zirconium (Zr) and the like, to absorb outgassed matter. Gettering material has also been utilized in FED technology, with one example shown in FIG. 2. Cathode plate 30, including the emitters (not shown) is separated from anode plate 32 by sealing walls 34. Spacers 36 are usually placed between the cathode and anode plates, to prevent the atmospheric pressure external to the display from distorting the anode plate after evacuation of FED. The cavity 42 between the plates is evacuated through the exhaust tube 38 by vacuum pumping means, and then sealed off to maintain a high vacuum in the display. Gettering material 40, in the prior art design of FIG. 2, is positioned within the exhaust tube 38. This provides a convenient means for heating, and thereby activating, the localized gettering source, after the exhaust tube 38 has been sealed off.
However, gettering materials localized in the exhaust tube are not very effective at absorbing volatile material from the FED cavity. The FED is usually large in size, on the order of 1-20 centimeters in width (denoted as L in FIG. 2), as compared to the small distance D between the cathode and anode plates of between about 100 to 1000 micrometers. The outgassed material is not very effectively removed due to the narrow passageway and remote location of the gettering material.
One method of providing improved gettering efficiency is described in U.S. Pat. No. 5,083,958 (Longo, et al.), in which additional interconnecting channels are formed between the base for the field emitters and the gate electrode, thus providing additional channels for the outgassed material to escape. However, the gettering material is formed on the peripheral inner walls of the FED, and are still a considerable distance from the outgassing surfaces. Local undesirable pressure increases can still occur during operation of this FED design, and gas conductance is low. In addition, extra space is required around the periphery of the display elements.
Another method of removing the outgassed materials is disclosed in U.S. Pat. No. 5,223,766 (Nakyama, et at.), in which the gate electrodes themselves are composed of non-evaporable gettering materials, such as alloys of tantalum (Ta), zirconium (Zr), titanium (Ti) and hafnium (Hf). Thus the getter is in close proximity to the anode surface from which outgassing occurs. However, this causes process compatibility problems in the fabrication of the display--since the etching rate of most gettering materials by hydrofluoric acid (HF) is about 10 times faster that of silicon oxide (SiO.sub.2), the gate electrodes will be seriously attacked by HF in the SiO2 cavity formation step (see FIG. 7).